At present, a twist drill used in machining operations comprises a lateral edge, a cutting blade, and a helical sub-edge platform, and a sub-edge substantially exhibits a single convex shape. Though a milling cutter has a structure in which its sub-edge and cutting edge are distributively set as three edges, four edges, five edges, etc., there is no great difference between its sub-edge and cutting edge and those of the twist drill. Such twist drill and milling cutter, when drilling (milling) work pieces of relatively high hardness, are liable to generate a large amount of heat, cause the hardness of the cutter heads to be low, and result in the wear of the cutter heads, and at same time such cutter heads have a defect that they are liable to be broken. Further, since the cutting edges on both sides of the cutter head are symmetrical in operation, and generally the cutting amount ranges only from about dozens of micrometers to 1 mm, the height of the repairing and grinding angle is difficult to be grasped when being repaired and ground. In addition, in those twist drills and milling cutters, since the center position of the cutter head can not be easily grasped, the problems are liable to be caused that the location is not accurate, that the precision of the machined hole is not qualified, that waste products is easily resulted, and at the same time a thinner work piece, when being machined, can be raised up to cause injury and accidents.